Maximum Zs

Max permitted Zs = 0.8 × tabulated Zs

Look up the tabulated maximum earth fault loop impedance for a protective device from BS 7671 Table 41.3, apply the 0.8 rule-of-thumb correction for conductor operating temperature, and check a measured reading against it.

Reference

BS 7671 Table 41.3 (0.4 s, 230 V); correction per Reg 411.4.5 / Appendix 3

How this works

BS 7671 Table 41.3 tabulates the maximum earth fault loop impedance that still lets a protective device disconnect within the required time. Those figures assume the conductors are at ambient temperature.

In service the conductors are warmer, their resistance is higher, and the measured loop impedance you take on a cold circuit will be lower than the value that circuit will actually reach under load. The common rule of thumb, which this calculator applies, is to compare your measured reading against 0.8 × the tabulated value, leaving 20% headroom for that temperature rise.

The rule of thumb is not the only permitted method. BS 7671 Appendix 3 and IET Guidance Note 3 also allow you to correct the measured value using the conductor's actual temperature, which can be less conservative. Where a result is marginal, use the full correction rather than the 0.8 factor.

Table 41.3 — maximum Zs, 0.4 s, 230 V

Rating	Type B (Ω)	Type C (Ω)	Type D (Ω)
3 A	14.57	—	—
6 A	7.28	3.64	1.82
10 A	4.37	2.19	1.09
16 A	2.73	1.37	0.68
20 A	2.19	1.09	0.55
25 A	1.75	0.87	0.44
32 A	1.37	0.68	0.34
40 A	1.09	0.55	0.27
50 A	0.87	0.44	0.22
63 A	0.69	0.35	0.17
80 A	0.55	0.27	0.14
100 A	0.44	0.22	0.11
125 A	0.35	0.17	0.09

Type D figures are the 0.4 s values. Values transcribed from BS 7671:2018+A4:2026 Chapter 41.

Limitations

This calculator is a revision and cross-checking aid. It does not replace the published tables in BS 7671, the specific requirements of the installation in front of you, or the judgement of a competent person. Always verify against the current edition of the standard before relying on a result.